Transparent conductive-oxide coatings are used in many types of electronics devices where an electrically conductive channel is formed on a translucent substrate such as a flat panel display screen or photo-voltaic solar cell. One type of transparent conductive-oxide coating is made from tin oxide (SnO2) that is formed on the translucent substrate from a gas or liquid precursor containing tin. The oxygen component may already be present in the tin precursor, or may be supplied separately by exposing the tin precursor to an oxygen-containing gas such as air. The fluidity of the tin precursor permits efficient coating and patterning of the tin oxide film on the substrate.
The conductivity of tin oxide films are significantly increased when specific dopants are included when the film is being formed. Originally, antimony was added to tin oxide films to increase their conductivity. Later it was discovered that fluorine also increased conductivity, while also maintaining the high transparency of the tin oxide film. The combination of improved electrical conductivity and transparency of fluorine-doped tin oxide films (FTOs) favor these films for solar cell applications. Fluorine-doping is also used to improve the electrical, optical, and other properties of materials such as zinc oxides, cadmium oxides, silicon oxides, indium-tin oxides, Pb—Zr—Ti oxides and other piezo-electric ceramics, carbon, silicon nitrides, and super-conducting materials such as mercury-barium oxides, and mercury-barium-copper oxides.
However, there are still challenges for effectively and efficiently incorporating a fluorine dopant in tin oxide thin films. Some organic-fluorine precursors can contaminate the FTO with excess carbon that can reduce both the electrical conductivity and transparency of the film. The combination of fluorine with oxygen, or other halogens (e.g., chlorine) can create oxidized build-up and corrosive deposits on the substrate. Thus, the choice of the fluorine dopant can have unpredictable and deleterious effects on the deposited FTO film.
Fluorine-containing precursors can also be highly toxic and corrosive, creating safety and environmental concerns about their use on a commercial scale. The health, safety, and handling risks of molecular fluorine (F2) and hydrogen fluoride (HF) are well known. Many chloro- and bromo-fluorocarbons have been banned for commercial uses due to their stratospheric ozone depleting and global warming properties when released into the atmosphere.
Corrosive fluorine-containing precursors can also rapidly contaminate FTO film fabrication equipment so that it requires frequent and time-consuming cleaning and refurbishing. Cleaning procedures often involve disassembling the equipment and contacting it with specialized cleaning solutions. This not only creates long delays before the equipment is operational again, but also generates a potentially toxic and corrosive source of spent cleaning fluids that is expensive to dispose. Thus, there is a need for effective fluorine dopant precursors in FTO film production with reduced health, handling and environmental risks compared currently used precursors. Such precursors, as well as their use in fabricating FTO films, are described here.